In conventional or “wet” lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink receptive regions accept the ink and repel the water. The ink is transferred to the surface of a material upon which the image is to be reproduced. For example, the ink can be first transferred to an intermediate blanket that in turn is used to transfer the ink to the surface of the material upon which the image is to be reproduced.
Imageable elements useful to prepare lithographic printing plates typically comprise an imageable layer applied over the hydrophilic surface of a substrate. The imageable layer includes one or more radiation-sensitive components that can be dispersed in a suitable binder. Alternatively, the radiation-sensitive component can also be the binder material. Following imaging, either the imaged regions or the non-imaged regions of the imageable layer are removed by a suitable developer, revealing the underlying hydrophilic surface of the substrate. If the imaged regions are removed, the element is considered as positive-working. Conversely, if the non-imaged regions are removed, the element is considered as negative-working. In each instance, the regions of the imageable layer (that is, the image areas) that remain are ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water and aqueous solutions, typically a fountain solution, and repel ink.
Imaging of the imageable element with ultraviolet and/or visible radiation is typically carried out through a mask that has clear and opaque regions. Imaging takes place in the regions under the clear regions of the mask but does not occur in the regions under the opaque mask regions. If corrections are needed in the final image, a new mask must be made. This is a time-consuming process. In addition, dimensions of the mask may change slightly due to changes in temperature and humidity. Thus, the same mask, when used at different times or in different environments, may give different results and could cause registration problems.
Direct digital imaging has obviated the need for imaging through a mask and is becoming increasingly important in the printing industry. Imageable elements for the preparation of lithographic printing plates have been developed for use with infrared lasers. Thermally imageable, multi-layer elements are described, for example, U.S. Pat. No. 6,294,311 (Shimazu et al.), U.S. Pat. No. 6,352,812 (Shimazu et al.), U.S. Pat. No. 6,593,055 (Shimazu et al.), U.S. Pat. No. 6,352,811 (Patel et al.), U.S. Pat. No. 6,358,669 (Savariar-Hauck et al.), and U.S. Pat. No. 6,528,228 (Savariar-Hauck et al.), U.S. Patent Application Publication 2004/0067432 A1 (Kitson et al.). U.S. Patent Application Publication 2005/0037280 (Loccufier et al.) describes heat-sensitive printing plate precursors that comprise a phenolic developer-soluble polymer and an infrared radiation absorbing agent in the same layer.
Problem to be Solved
Multilayer lithographic printing plates usually include one or more IR-sensitive layers that are removed using an alkaline developer after imaging. Such layers are usually top layers and can be composed of various phenolic resins such as novolac resins, resole resins, and various hydroxy-substituted acrylates as described for example in the publications noted above.
Imageable elements having topcoats comprising cyclic olefin copolymers are described in U.S. Pat. No. 6,969,570 (Kitson). Further, U.S. Patent Application Publication 2004/0137366 (Kawauchi et al.) describes the use of copolymers comprising pendant carboxy groups or maleic anhydride in top layers of heat-sensitive positive-working elements to improve scratch resistance and development latitude. These copolymers can be developed in relatively “weak” developers that may be considered more environmentally “friendly”.
U.S. Pat. No. 6,152,036 (Verschueren et al.) describes the use of hardened epoxy resins in the top layers of positive-working imaging elements. Crosslinking the top layer is said to improve physical and chemical resistance.
There is a desire in the industry to provide positive working imaging elements that have high image resolution (or high discrimination between imaged and non-imaged regions). In addition, there is a need for improved imaging speed and more rapid and complete removal of imaged regions. In many instances, what provides one of these properties worsens others. Moreover, there is a need for imageable elements that can be processed in “cleaner” seasoned developers in which polymeric materials removed during processing are more fully soluble or dispersible in the developing solutions, thereby reducing residue or sediment in the developer tanks and the need for developer filtration.